Modulation of spinal excitability by a sub-threshold stimulation of M1 area during muscle lengthening

It is well known that the H-reflex amplitude decreases during passive muscle lengthening in comparison with passive shortening. However, this decrease in spinal synaptic efficacy observed during passive lengthening seems to be lesser during eccentric voluntary contraction. The aim of the present study was to examine whether spinal excitability during lengthening condition could be modulated by magnetic brain stimulation. H reflexes of the triceps surae muscles were elicited on 10 young healthy subjects, and conditioned by a sub-threshold transcranial magnetic stimulation (TMS). The conditioning stimulation was applied over the M1 area of triceps surae muscles at an intensity below motor threshold with a conditioning-test interval of 5 ms. Conditioned and non-conditioned H-reflexes were elicited at rest, during passive lengthening and shortening, and during submaximal contractions (concentric, eccentric and isometric). During passive and active lengthening, H reflexes conditioned by a sub-threshold TMS pulse increased on average by 50% compared with non-conditioned responses. No significant effect was found during isometric and concentric conditions. Activation of the corticospinal pathway would partially cancel inhibitions caused by muscle stretch, and according to the time-delayed effect, this result suggested the existence of a specific polysynaptic pathway. In additional experiments, H responses were conditioned by cervico-medullary stimulations, showing that the modulation described by the previous results involves subcortical mechanisms. This study provides further evidences that the modulation of the final cortico-spinal command reaching the muscle depends on a central mechanism that controls peripheral input, such as Ia afference discharge during lengthening.